This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Polyketides include many important drugs with cholesterol-lowering (such as lovastatin), anticancer (such as doxorubicin and bleomysin) and antibiotic activities (such as tetracycline and erythromycin) with potential HIV applications. Polyketide synthase (PKS) is a multi-domain enzyme complex that produces a huge variety of polyketides via a controlled variation of building blocks and modification reactions, including chain reduction and cyclization. There is a critical gap in the structure and molecular basis of PKS activity. To solve the PKS complex structures, we have crystallized several PKS complexes that diffract to 1.7 ? 2.8 [unreadable]. Because of the size of these complexes, we could not solve them by molecular replacement using single-domain structures. We have already crystallized the PKS complexes as Se, Au and Pt derivatives that diffract in-house to 2.2 ? 4.0 [unreadable]. Therefore, beamtime at SSRL will have a great impact by helping us bring this exciting project to the finish line. As an outcome of the proposed investigations, we expect to determine all three multi-domain complex structures and determine the importance of protein-protein interaction on the polyketide cyclization/termination specificity. The polyketide ring is vital for the antibiotic and anticancer properties of polyketides. The research proposed in this application is significant, because its outcome allows us to elucidate how polyketides are specifically cyclized and terminated at a molecular level that has not been visualized before. In the long run, the result from this proposal will have a significant positive impact on the public health by providing medically useful chemicals via structure-directed mutants of PKS.